1. Field of Invention
The invention relates to the field of security systems, and more particularly to a contact tamper switch for detecting attempts to tamper with components of a security system.
2. Description of Related Art
In the field of security systems it is known to provide some means to detect attempts to tamper with the system, for example as an attempt to disable or defeat it. In particular, unauthorized access to system components may be presumed as attacks on the system. In particular, components of the security system may be accessible by potential intruders while the security system is in an unarmed state. As an example, the system component may be located in a room open to the public during normal hours, while the security system is unarmed.
In one embodiment, a circuit board, for example a printed circuit board (PCB), as an element of a security system component, is provided with a pressure-sensitive switch. A cover or enclosure of the security system component can act on the switch when the cover or enclosure is closed. Alternately or additionally, a pressure-sensitive switch may act on the surface the security system component is mounted to. Opening the cover or enclosure, and/or removing the system component from the surface it is mounted to, releases the pressure on the switch, which then changes state to indicate the presumed attack.
This approach has certain drawbacks, however. Among these is the localized pressure on the switch causing uneven stress in the PCB and/or the enclosure or cover. This may lead to fracture or failure with repeated opening and closing of the switch, cover, and/or enclosure. Alternately, the material of the enclosure, for example plastic, may deform or ‘creep’ due to long term closure of the switch. Accordingly, the pressure sensitive switch may no longer be reliably closed due to the material creep. Additionally, the surface to which the system component is mounted may be uneven, and not provide a reliable surface to actuate the pressure sensitive switch.
An alternate tamper detection means comprises a reed switch and magnet arrangement. The reed switch changes state in proximity of the magnet, and the two are located in proximity with one another when the component is mounted to the wall and/or the cover or enclosure are closed. When the moved away from the magnet, typically by opening the cover or enclosure, the reed switch changes state to indicate the presumed attack. At least one drawback of this arrangement is the vulnerability of the reed switch. If a larger magnetic field with the proper polar orientation is placed in the vicinity of the reed switch, the reed switch will not detect loss of proximity with the magnet, which indicates the component may have been disturbed as part of a presumed attack.